1. Field of the Invention
The present invention relates to a projection display apparatus having a semiconductor laser as a light source, such as a projection television, and particularly to a screen unit, in which a thin sheet glass is sandwiched between a Fresnel lens sheet and a lenticular lens sheet, and a projection display apparatus that includes the screen unit.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-open No. 2001-154274 (page 2 and FIG. 3), there is generally known a technique in which a sheet glass is sandwiched between two lens sheets, which are a Fresnel lens sheet and a lenticular lens sheet. These lens sheets are warped in a concave or convex manner by heat treatment and laminated to be closely attached to the sheet glass to form a support structure, which prevents the lens sheets from waving in a concave or convex manner due to expansion or contraction of a screen according to environment changes in an ambient temperature or humidity.
In a projection television that includes a transmission screen with a high incidence angle, which realizes reduction in the depth of the product, images are deformed even by small deformation on an imaging surface of a Fresnel lens sheet. Accordingly, usage of a screen formed by bonding two lens sheets to a glass substrate with an ultraviolet curable resin or a thermoset resin is also generally known.
In a conventional screen unit, when an image composed of light modulated according to an image signal is displayed on a screen, scintillation in which bright spots and dark spots are distributed randomly sometimes occurs due to light interference, which may make observers uncomfortable. Because laser light has high coherence, scintillation particularly easily occurs. Accordingly, a method of vibrating at least one of two screens to suppress scintillation is proposed (Japanese Patent Application Laid-open No. 2005-107150 (page 7 and FIG. 3)), and also there is generally known a method of integrally forming a diffuse material inside of the two screens to provide an imaging surface to each of the Fresnel lens sheet and the lenticular lens sheet to suppress scintillation.
However, these conventional techniques leave some problems unresolved.
The first problem is as follows. The lens sheets, which are to be laminated on the thin sheet glass, are curved in cylindrically convex shapes by heat treatment to provide curvature factors to the lens sheets, the curved surfaces of which correspond to rotation surfaces around horizontal axes. In doing so, the axes of curved surfaces of the two lens sheet are set to be the same (both axes are horizontal). Therefore, when three sheets are laminated, reaction force in directions away from the thin sheet glass is generated on upper and lower sides of a screen, which makes an operation to fit the sheets into a groove formed on a screen holding unit of the screen unit troublesome. Before fitting the sheets into the screen holding unit, measures of holding four side edges with U-shaped clips, tapes, or the like are sometimes taken. However, even with this configuration, the clips or tapes are easily unhooked or torn.
The second problem is as follows. If there is a clearance from the holding unit along the periphery of the screen when the screen unit is fitted into a frame rim, restoring force of the curved lens sheets causes deformation in peripheral portions, and the periphery of a projected image is deformed. In contrast, if the holding unit is firmly pressed, movement caused by expansion or contraction of the lens sheets according to changes in a temperature or humidity is hindered. Therefore, central portions of the lens sheets are bulged and the center of the projected image is deformed.
The third problem is as follows. The bonding of the Fresnel lens sheet and the lenticular lens sheet to the glass substrate or the vibration of one of the lens sheets against the scintillation makes the method of manufacturing a screen or the configuration of the screen complicated, which makes screen holding devices expensive.